1. Field of the Invention
The present invention generally relates to electrical circuits and their fabrication. More particularly, this invention relates to a process for forming thin-film metal resistors that can have high resistance without increasing parasitic series inductance, and are therefore highly desirable for use in multilayer high-density electronic circuits.
2. Description of the Prior Art
Thin-film resistors formed of such metal-based materials as nickel-phosphorus, nickel-chromium, chromium silicide and tantalum nitride have been employed in multilayer hybrid electronic circuits. Thin-film metal materials generally exhibit good resistor properties, such as stability and ease of processing, but are limited to low sheet resistance, typically on the order of 100 ohms/square or less. Many resistors in a typical electrical circuit have resistance values in the kilo-ohm range. While such resistors can be fabricated using a thin-film metal resistor material, the low sheet resistance of the material necessitates that the resistors be ten to one hundred squares in size, e.g., about five mils (127 micrometers) wide and about fifty to five hundred mils (1.27 to 12.7 millimeters) long. Resistors of this size pose several problems. First, they have high parasitic series inductance, which degrades the resistor""s performance for high frequency applications. Secondly, they encumber an excessive amount of board area. By occupying so much board area in a multilayer high-density board construction, the resistors greatly aggravate the problem of unwanted z-axis interactions with circuit elements in overlying and underlying circuit layers.
As an alternative, screen-printed polymer thick-film (PTF) materials offer higher sheet resistances than thin-film resistive metals. However, PTF materials are less stable under environmental stress, and are not as compatible with large format printed circuit board fabrications. Accordingly, it would be desirable if a method were available for producing resistors for multilayer high-density printed circuit boards that had the property advantages of thin-film metal resistor materials, but avoided the disadvantages associated with the use of such materials when used to form resistors having resistance values of 1000 ohms or more.
According to the present invention, there is provided a thin-film metal resistor suitable for a multilayer printed circuit board, and a method for its fabrication. The resistor of this invention generally has a multilayer construction, with the individual layers of the resistor being self-aligned with each other so that a negative mutual inductance is produced that very nearly cancels out the self-inductance of each resistor layer. As a result, the resistor has a very low net parasitic inductance. In addition, the multilayer construction of the resistor reduces the area of the circuit board required to accommodate the resistor, and as a result reduces the problem of parasitic interactions with other circuit elements on other layers of the circuit board.
The method of this invention generally entails forming a first resistive film on a substrate, forming a dielectric layer that overlies the first resistive film, forming a second resistive film on the dielectric layer so that the first resistive film is superimposed by the second resistive filmxe2x80x94more particularly, the first and second resistive films are aligned with each other, so that their perimeters superimpose each other. This aspect of the invention is promoted by forming the first and second resistive films to be self-aligned by using a single mask to define their shapes. Adjacent portions of the first and second resistive films are then electrically interconnected, so that a resistor path is defined that starts at a first portion of the first resistive film, follows the first resistive film to its electrically interconnected portion, continues to the adjacent portion of the second resistive film through the electrical interconnect therebetween, and then follows the second resistive film to a portion thereof adjacent the first portion of the first resistive film.
According to the above, the thin-film resistor produced by the method of this invention is characterized by at least two superimposed resistive films that are electrically interconnected so that the resistive path through the resistor loops back, producing a negative mutual inductance that very nearly cancels out the self-inductance of the individual resistive films.
Other objects and advantages of this invention will be better appreciated from the following detailed description.